In U.S. Pat. No. 3,624,004 a process is disclosed whereby a pyrolyzate, which is made by heating above the salt decomposition temperature an aromatic acid and at least enough of an electrolyte, e.g., potassium hydroxide, to form a salt of the aromatic acid, is subjected to controlled oxidative activation above 1,300.degree. F. in the presence of carbon dioxide to produce a high surface area, low bulk density active carbon.
In U.S. Pat. No. 3,642,657, processes are disclosed in which low surface area active carbons are made by decarboxylation of petroleum coke acids and such decarboxylated materials are further heated above about 1100.degree. F. with a solute, for example potassium hydroxide, in the presence of a hydrogen halide, carbon monoxide or carbon dioxide, to form a high surface area, low bulk density active carbon.
In U.S. Pat. No. 3,817,874, a process is set out for increasing the surface area of a low to intermediate surface area active carbon by heating such carbon above about 1100.degree. F. with sodium or potassium hydroxide in the presence of carbon dioxide. The process produces high surface area active carbons of moderate bulk density.
In U.S. Pat. No. 3,833,514, a process is set forth to produce high surface area active carbons by heating the salt of an aromatic acid admixed with electrolyte, e.g., potassium hydroxide, above the decomposition temperature of the salt. The process produces active carbons of low bulk density.
Although the active carbons produced in the above processes are of high effective surface area as measured by BET and of generally good properties several process and product deficiencies are noteworthy: (1) the processes are of less than maximum yield based upon carbonaceous feed consumed when the more economical hydrous alkali is used, (2) when coke acid carbonaceous feeds are used the total process requires extra processing steps, and (3) the feed combination of alkali and carbonaceous material during calcination forms a sticky, viscous mass which is very difficult to handle in commercial operations because of adhesion to the walls and plugging of the calcination (activation) vessel. The source of problems (1) and (3) when potassium hydroxide is employed appears to lie in use of the more economical hydrous alkali for continuous processes wherein spent carbon wash solutions are evaporated and treated to recover alkali for recycle purposes. More particularly, yield loss is thought due to oxidative attack by water vapor during high temperature calcination of the alkali-carbonaceous feed combination.
Further, the bulk density of the active carbon made from an aromatic acid or petroleum coke acid is lower than desirable for many commercial applications and the Total Organic Carbon Index per volume of carbon used, an important and industry recognized measure of the power of the active carbon to remove organics from water effluents, is far from maximized. Also, the effective BET surface area of such carbons while high is still not maximized.
Now, a higher yield process has been developed which uses hydrous potassium hydroxide and substantially eliminates the processing problems referred to above. Further, the process can produce a carbon with the unique combination of good bulk density, very high surface area and excellent Total Organic Carbon Index. The key to the new process is to carry out the heating of the hydrous alkali-carbonaceous feed combination in two steps wherein the first, lower temperature step is carried out with agitation and dehydrates the feed charge prior to the second, higher temperature activation step. The key to the new combination of product properties is the production of a carbon structure in which a substantially larger portion of the surface is substantially cage-like, which cage-like structure exhibits properties of microporosity.